There exists a wide variety of devices, generically referred to as Microwave Proximity Sensors, which are capable of detecting the presence or movement of an object at a distance. These devices are all responsive to changes in the standing wave pattern surrounding a microwave source caused by interference between microwaves reflected from an object and the directly radiated signal.
The size and complexity of such sensors vary widely depending upon the requirements for range and precision. On the one hand, radar sets have ranges exceeding 1000 miles and are able to track numerous targets simultaneously and determine the range and azimuth of each one. On the other hand, small sensors intended as intrusion detectors for burglar alarm systems, have a maximum range of perhaps 100 feet and limited capability for determining range or direction.
The longer range sensors generally employ vacuum tube type microwave oscillators such as the Klystron or Magnetron, coupled to highly directional transmitting antennas, an example of which is the familiar parabolic "dish" reflector. Short range sensors often employ solid state microwave oscillators, such as the step-recovery diode or the IMPATT diode, in combination with a horn type radiator. The most inexpensive sensors, intended for use in security systems, often utilize microwave transistor oscillators coupled to very simple antennas such as the omnidirectional 1/4-wavelength stud or the only slightly more directional 1/2-wavelength folded dipole antenna. The cost of such sensors can range from many millions of dollars for long range radar sets to about $100 for the simplest intrusion detectors.
Apart from the usual applications in radar and sophisticated burglar alarm systems there are a number of consumer-oriented applications, such as automatic doorbells and automatic light switches, where a microwave proximity sensor would be useful. However, their use in these areas has been very limited to date, due to the comparatively high cost of even the simplest systems and the absence of a sensor having power requirements low enough to permit operation from batteries.
Existing microwave proximity sensors employing transistor oscillators utilize a single 1/4 to 1/2 length folded dipole antenna as both transmitting and receiving antennas. An example of a sensor of this type is described in U.S. Pat. No. 4,313,118 granted Jan. 26, 1982, to the Applicant herein. In such sensors, changes in the standing wave patterns surrounding the antenna caused by the approach of a person or object are reflected in changes in the total power requirements of the oscillator. Fluctuations in the power drain of the oscillator larger than a pre-set level are used to trigger external circuits, such as alarms. Such a microwave proximity sensor is satisfactory for its intended use in connection with detection of approaches of persons or objects, primarily for security purposes. In certain applications, however, it has been found desirable to place low limits on the total power drain of the system. The invention described herein responds to that need.